The most frequent cause of failure of components of machinery exposed to high abrasion or erosion is wear. The problem is exacerbated when abrasion or erosion is combined with corrosion such as occurs on fan blades used for conveying products of combustion in coal-fired power plants. Many types of coatings have been developed to protect against wear. The most widely used coatings are hard chromium, weld overlays, and thermal spray coatings. Thermal spray coatings are reviewed by Lech Pawlowski in The Science and Engineering of Thermal Spray Coatings. (John Wiley and Sons 1995).
Thermal spray coatings of hard particles are made by: Detonation Guns, High Velocity Oxygen Flame Spraying, Plasma Spraying, Wire Arc Spraying, and Flame Spraying. Of these, carbide coatings made with detonation guns are the most resistant to wear. With the exception of vacuum plasma spraying, which is used infrequently relative to other coating processes, all of these means for thermal spraying expose the powders being sprayed to oxygen or water vapor. This leads to metal oxide formation that is detrimental to the coating. They all depend upon mechanical entrainment for bonding the coating to substrates. In some instances, thermally-sprayed coatings are heat treated to enhance bonding. Geometries that can be coated by thermal spraying are limited to line of sight between the end of the spray nozzle and the surface being coated. Further, if the angle of impingement is not constant and at a relatively large value, non-uniform coatings are produced. Thermal spraying is limited to the range of composite structures that can be can be made. For example, spraying particles with a difference in particle diameters of 10 to 1 is difficult, because the powders segregate in the powder delivery systems.
Another process for applying coatings of carbide particles is described in U.S. Pat. No. 3,743,556. It is based upon the infiltration of braze into a layer of tungsten carbide particles. In that process, coatings are made by first applying cloth containing particles of tungsten carbide to a surface needing protection against wear. Another piece of cloth containing particles of braze alloy is placed over the layer of carbide particles. The substrate with the two layers of cloth is placed in an inert-atmosphere furnace and heated to the brazing temperature of the braze alloy. Braze infiltrates down into the carbide particles and brazes them to each other and to the substrate.
The principle disadvantage of the cloth process is the difficulty of making thin coatings less than 0.25 millimeters thick (0.01 inch). The process for making the cloth described in the patent is not amenable to producing such thin cloth. Further, the process is relatively costly because of the expense of making the cloth and manually applying the cloth to surfaces needing protection. While this process can apply very uniform coatings to a much wider spectrum of complex geometries than spray coatings, it is still limited to geometries that can be accessed with fingers or tools. Another disadvantage of the cloth process is seams. They often result in structural discontinuities in the carbide coating.
These disadvantages of both thermal spraying and the cloth methods for applying carbide containing coatings are overcome by the present invention.
The closest prior art is considered to be U.S. Pat. No. 3,743,556, which is hereby incorporated by reference. This patent discloses the infiltration process and commercially produced braze pastes for joining metals. While this patent teaches the use of mixtures of adhesives with tungsten powder and braze powder for applying layers of carbide and braze, it does not disclose the use of paints made in accordance with the present invention.
The rheology of braze pastes available from Wall Colmonoy Corporation, Madison Heights, Mich. would not result in stable carbide paints. Densities of the nickel/chromium braze alloys that are suspended in these pastes are around 8 grams per cubic centimeter, which is far less than the density of the suspended particles used in the paints of the present invention. The Wall Colmonoy product literature describes its braze pastes as having viscosities up to 400 centipoises.
In the prior art, grinding wheels are made by mixing hard particles with adhesives. But they are not formulated into paints that can be applied to vertical surfaces. They are cast into shapes of abrasive wheels. Another type of grinding wheel is made by brazing particles of diamonds or tungsten carbide onto the outer periphery of steel wheels. A variation of this is brazing larger particles of tungsten carbide to grippers for manipulating pipe and other objects. These grinding wheels, files, and grippers are made using a brazing technology in which the braze is placed between the surface being coated and the hard particles.
The inside of a pipe has been coated by the procedure described in U.S. Pat. No. 4,490,411. It describes distributing particles in a semi-fluidized state within the inside diameter of a tube as it is simultaneously heated and rotated. This has the problem of maintaining mixed powders of two different densities in the fluidized state. They tend to classify or separate by density, which inhibits the uniform distribution of the powders.
The present invention takes advantage of the best features of both the thermal spraying and cloth processes by using a process based upon paint technology. In a first embodiment of the invention, hardfacing particles and braze-alloy particles are made into separate paints. The hardfacing particle layer is first xe2x80x9cpaintedxe2x80x9d over the area of metal needing protection. Over that, a layer of braze is xe2x80x9cpainted.xe2x80x9d The surface thus coated is heated in a furnace in an inert atmosphere to a temperature that is above the melting (liquidus) temperature of the braze alloy. The braze alloy then infiltrates down into the layer of hardfacing particles and brazes (metallurgically bonds) them into a composite of hard particles in a matrix of braze alloy onto the substrate metal. The resulting coating is comprised of hard particles metallurgically bonded to the substrate metal. It poses resistance to abrasion and erosion far above that of tool steels, hard chromium, and most thermally sprayed carbide coatings. It can be applied to various shapes, including concave shapes, such as the inside diameters of long pipes, inside of pipe elbows, inside of pumps, valves, and onto other complex geometries that cannot be coated by thermal spraying or by the cloth-based coating processes.
The density of tungsten carbide, which is a preferred hardfacing particle, is about 15 grams per cubic centimeter. Other hardfacing particles used in the paints of this invention also have a density greater than 10 grams per cubic centimeter. Making layers of hardfacing particles and braze particles in the infiltration process for making carbide-containing coatings has been hampered by the difficulty of making stable paints containing such dense particles. Since the density of tungsten carbide is 14 to 15.4 times greater than that of water, the tungsten carbide has a great tendency to settle in paint formulations. We have discovered that the stability of a tungsten carbide paint is dependent upon the difference in viscosities of the paint at high and low shear rates of the paint. In other words, we want the paint to be very viscous at low shear rates, such as after it has been painted onto a substrate, so the hard particles will stay in place on the substrate, and we want the paint to be much less viscous at high shear rates, so that it can readily be sprayed onto the substrate. Tungsten carbide paints having low-shear viscosities below around 100,000 centipoises sag severely when applied to vertical surfaces. The high viscosity of the paint has to be achieved by increasing the viscosity of the adhesive used to make the paint rather than by adding more carbide particles.
In a second embodiment of the invention, a layer of adhesive is applied to a metal substrate, and hardfacing particles are applied to that adhesive layer. After drying, another layer of adhesive is applied over the adhered hard particles. Braze powder is then applied to the layer of wet adhesive thus forming a layer of braze particles in juxtaposition to the layer of hard particles. Heating in an inert atmosphere then causes metallurgical fusion, which produces a composite of hard particles in a matrix of braze metallurgically bonded to the substrate metal.
In a third embodiment, a hardfacing alloy powder containing precipitated intermetallic hard compounds is made into a paint and applied to the surface being protected. After drying, it is then heated in an inert atmosphere to a temperature above the solidus of the hardfacing alloy to form a fully dense coating of the hardfacing alloy metallurgically bonded to the substrate.
In a fourth embodiment, hardfacing particles and a hardfacing braze alloy powder are made into a paint and applied to the surface being protected. It is then dried and heated in an inert atmosphere to a temperature above the solidus of the hardfacing alloy to effect metallurgical bonding of hardfacing particles to the substrate by the hardfacing alloy.